In existing active matrix organic light-emitting diode (AMOLED) display panels, there exist two types of driving mechanisms: analog driving and pulse width modulation (PWM) driving.
In an AMOLED pixel circuit employing the analog driving, a current flowing through an OLED of the pixel is controlled according to a display grayscale. Since the OLED device operates infrequently at a maximum current, it benefits from a prolonged lifetime. However, for this type of driving, a driving device (e.g. a thin film transistor, TFT) generally has to suffer a large divisional voltage from voltage modulation, which results in ineffective power consumption and hence low efficiency. Additionally, the need for a precise control over the current generally leads to a complicated, associated pixel circuit.
By contrast, in an AMOLED pixel circuit employing the pulse width modulation driving, the TFT operates in a linear region, which results in a small voltage drop and hence low ineffective power consumption, thereby meeting the requirement of the existing display device for low power consumption. However, the pulse width modulation driving technology divides an image frame period into a plurality of sub-frames, and controls a total pulse width of a driving pulse being ON within one image frame period by driving the light-emitting device of the pixel to switch ON/OFF within each sub-frame, so as to achieve a grayscale control (i.e., outputting digits “0” or “1” discretely, which can produce a similar effect to an analog output when a refreshing frequency is sufficiently high). Thus, if the pulse width modulation driving is applied directly to drive the pixel circuit, the frequency of data control signal refreshing and driving actions has to be much greater than the display frame frequency, which is difficult to implement in circuits. Moreover, with the OLED of the pixel operating only in either an ON state with a maximum current or an OFF state with a zero current, the operation current is large during the ON state of the OLED of the pixel, which easily results in a reduced service life of the OLED of the pixel.
Therefore, there is a need for an improved pixel circuit, display panel and driving method thereof.